PROBLEMS OF MODERN SCIENCE 



extremities, they are occupied by a considerable 

 bulk of growing cartilage, so large that for a time 

 bone-formation and cartilage-growth keep pace 

 with one another, the bone-forming tissue being 

 seemingly incapable of overcoming the growing 

 cartilage and occupying the ends of the bone. 

 After a time, and this occurs at a comparatively 

 late stage in the history of the thigh-bone, the 

 bulky cartilaginous extremities are affected by 

 a series of changes exactly comparable with the 

 changes which affected the central part of the 

 shaft in the early beginning of bone-formation. 

 A spot centrally situated in the cartilaginous mass 

 exhibits degenerative changes, bone-forming tissue 

 invades from without, the cartilage disappears, 

 bone taking its place, and the process spreads 

 throughout the mass. 



The period of life at which this late indepen- 

 dent bone-building occurs in the extremities of 

 any particular bone, such as the thigh-bone, seems 

 to be fairly definite, but the process takes place at 

 widely different periods in the different bones of 

 the skeleton. The circumstances which are to be 

 held responsible factors determining the degenera- 

 tion of the cartilage in the outlying parts of a bone 

 and its replacement by bony tissue are not fully 

 understood. All that can be said is that, in most 

 cases, it seems to be a reaction to functional neces- 

 sity, endowing those parts of the bone with the 

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